A plurality of clutches and brakes (hereinafter, represented simply by clutches) which are operated to depress frictional plates under pressure by means of a hydraulic pressure are installed within a gear train in an automatic transmission. Thus, a plurality of gear shift stages are achieved according to a combination of engagements of the clutches and/or releases thereof. For example, a low clutch (LOW/C), a high clutch (HIGH/C), and so forth are installed. At a forward first speed, the low clutch is engaged. At a high-speed third speed, the low clutch is released and the high clutch is engaged.
Each clutch is provided with a hydraulic pressure chamber and a piston which strokes according to a working hydraulic pressure supplied to the hydraulic pressure chamber. An operation process of each clutch includes: a stage of a stroke end at a free running interval until the piston is contacted on the frictional plate; and a torque transmission control stage at which the piston actually depresses the frictional plate under pressure and the working hydraulic pressure is controlled to be varied to a maximum engagement pressure in order to smoothly vary the contact state of the piston and the frictional plate from a slip state to a completely engaged state.
In a vehicle in which such an automatic transmission as described above is mounted, in order to improve a fuel economy and to reduce a quantity of exhaust gas, an idling stop function is conventionally equipped in which an engine is automatically stopped in a case where the vehicle is temporarily stopped due to a signal wait at a traffic intersection during a traveling of the vehicle or in a case where the vehicle is waiting for a passage of a train at a railroad crossing and is, thereafter, started with the engine re-started when a predetermined condition is established.
Incidentally, the above-described clutch is engaged and released using the hydraulic pressure through a mechanical oil pump driven by means of an engine power. However, since the operation of the mechanical oil pump is stopped during the engine stop of the idling stop, the required hydraulic pressure cannot immediately be obtained at the time of the restart of the vehicle.
In addition, in one of the clutches to be engaged at a first speed for the re-start of the vehicle, the working oil of the hydraulic pressure chamber is drained during the idling stop. The oil quantity for the stroke end stage is demanded. Hence, such a problem that a desired response characteristic cannot be obtained at the time of restart after the idling stop is raised. A demand on a response characteristic is, specially, large in a case where the restart is a forward direction of the vehicle.
Therefore, for example, in a Japanese Patent Application First Publication (tokkai) No. 2002-115755, a hydraulic pressure control apparatus has been proposed in which an electrically driven oil pump is additionally installed in the automatic transmission and the hydraulic pressures from these respective hydraulic pressures are supplied to the starting clutch by a combination of an electrically driven oil pump and an accumulator within a hydraulic pressure circuit.